Not Applicable
1. Field of the Invention
This invention pertains generally to pixelated detector imaging circuits, and more particularly to a method and apparatus for selecting an input channel with the highest signal amplitude from an array of detector input channels.
2. Description of the Background Art
Multi-channel solid state readouts are widely used in image detection 5 applications where each pixel in a detector array is sensed independently from one another. Such readouts typically receive inputs from detector arrays and process the inputs to identify and selectively pass an analog signal corresponding to the highest amplitude input signal received. Within these systems the identification of a single cell containing the signal is resolved despite being constrained with signal levels which are not significantly higher than the noise level, and the existence of spurious signals within the other detector cells.
For example, a multichannel readout is commonly used in connection with a pixelated gamma camera which detects 140 keV photons resulting from positron annihilation. Such a detector couples a photodiode array to an array of scintillator crystals. Gamma ray interaction within one crystal of the scintillator array causes the appropriate cell of the photodiode array to produce a signal which is measured to determine in which crystal cell the interaction occurred and whether the energy deposited in that cell was consistent with a 140 keV photon.
Rapid sensing of the interaction within the scintillator array is required so that events are correctly registered and not missed. One accepted approach to providing for rapid sensing has been to feed the output from each detector cell into a multiplexer and a xe2x80x9cwinner-take-allxe2x80x9d (WTA) circuit that identifies the highest amplitude signal and activates the multiplexer to pass that signal to the output. Since the device reads the detector cells in parallel to determine the highest amplitude cell, it has the capability of providing faster cycle times than a system which reads detector outputs serially.
Unfortunately creating a workable integrated circuit that can receive inputs from more than approximately sixteen detectors has posed a challenge. Mixing analog and digital circuitry within an IC layout causes moderate to severe noise problems that reduce the accuracy of the results and create erratic switching responses. In addition, the incorporation of a large multi-channel solid-state detector readout into a system brings about serious difficulties relating to control and testing issues.
Those skilled in the art will appreciate, therefore, that mixing analog and digital circuitry in an integrated circuit created noise or coupling problems that are very difficult to solve. While noise reduction has traditionally been addressed by careful circuit layout, there is a large penalty for failure due to the time and expense involved in integrated circuit design. Therefore, a need exists for a multi-channel solid-state detector readout circuit that allows parallel reading of the detector cells and overcomes noise, erratic response, and integration problems. The present invention satisfies those needs, as well as others, and overcomes the deficiencies of previously developed solutions.
The present invention is a multi-channel detector readout method and circuit that is capable of tracking the highest intensity input within a detector array. The circuit, which is preferably implemented as an integrated circuit, can be easily incorporated within a detection system such as a gamma camera system. The tracking is provided by sensing and multiplexing circuitry wherein a single detector cell input, which has been found to have the highest amplitude, is selected from a series of detector cell inputs and routed to an output along with a digital identifier. The integrated circuit employs noise reduction circuitry and techniques which reduces erratic switching effects and incorporates a number of features which simplify device integration within a system.
By way of example, and not of limitation, according to an aspect of the invention the digital part of circuitry is turned off while the amplifiers are is active to reduce noise in the amplifiers. According to another aspect of the invention, an analog sample and hold circuit is employed and externally triggered to receive data. According to a still further aspect of the invention, a winner take all circuit is employed to always select the channel with the largest output. When there is no input, however, there is random noise on each channel so the channel with the largest output may be a noise channel. By adding a threshold circuit with a bias higher than the noise level, when no input signal is applied, the threshold output is selected as the winner. As a result, the digital outputs do not switch, thereby eliminating switching noise. For example, in a 64-channel winter takes all circuit, there are 64 cells. A 65th cell can be added and its input connected to an adjustable voltage source instead of an amplifier. The 65th cell will always be the winner if the voltage input to that cell from the adjustable voltage source (threshold) is greater than any of the other input voltages. Therefore, switching noise can be reduced or eliminated when there are no input signals by setting the threshold above the noise voltage.
An object of the invention is to provide input channel selection based on absolute and relative signal amplitude.
An object of the invention is to provide a multi-channel detector readout having higher noise immunity.
Another object of the invention is to provide a digital output corresponding with the selected channel.
Another object of the invention is to provide an external control mechanism wherein the routing of input channels according to the highest intensity signal may be over-ridden by external command.
Another object of the invention is to provide a selection mechanism capable of high-speed operation.
Another object of the invention is to provide for testing of the multi-channel detector readout wherein the inputs lines and selection process may be verified without the need of disconnecting the integrated circuit.
Another object of the invention is to provide a circuit wherein the input lines may be individually conditioned prior to receipt by the input line selection circuitry.
Another object of the invention is to provide the capability of channel by channel masking of the input lines during the selection process so that faulty inputs may be masked out rather than rendering the entire circuit inoperative.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.